The paper discusses the issue of the modelling of strains and stresses resulting from heating and cooling processes of components in power plants. The main purpose of the work is to determine the mechanical behavior of power plant components operating under mechanical and thermal loading. The finite element method (FEM) has been used to evaluate the temperature and stresses changes in components as a function of time. Temperature fields in the components of power plants are dependent, among parameters, on variable heat-transfer conditions between components and the fluid medium, that may change its condition, flowing inside them. For this reason, an evaluation of the temperature field and the consequent stress fields requires the use of heat-transfer coefficients as time-dependent variables and techniques for determining appropriate values for these coefficients should be used. The methodology that combines computer modelling of the temperature fields with its measurements performed at selected points of the pipelines may be used in this case. The graphs of stress changes as a function of time have been determined for the chosen plant components. The influence of the heat transfer conditions on the temperature fields and mechanical behavior of components in question have been discussed.
This paper discusses the issue of the modeling of strains and stresses resulting from the heating and cooling processes of components in power plants. The main purpose of this work was to determine the mechanical behavior of power plant components operating under mechanical and thermal loading. The finite element method has been used to evaluate the temperature and stress changes in components as a function of time. The temperature fields in the components of power plants are dependent, apart from other relevant parameters, on variable heat-transfer conditions between these components and the fluid medium (which may change its state) flowing inside them. For this reason an evaluation of the temperature field and the consequent stress fields requires the consideration of heat-transfer coefficients as time-dependent variables, which in turn calls for suitable techniques for the determination of appropriate values for these coefficients. Methodology that combines computer modeling of the temperature fields with temperature measurements performed at selected points of the pipelines may be used in this case. It is readily apparent from the stress-versus-time graphs that under unsteady operating conditions the components analyzed in this study, especially in the case of boiler restarts, may operate with transient thermal stresses that sometimes reach values higher than a yield point. Consequently, a thermo-mechanical fatigue process takes place in the materials of the components in question. Local stress-strain diagrams for the selected points of the plant components describe this kind of fatigue. These diagrams characterize the intensity of the process and are necessary when the fatigue life is predicted. Such diagrams are part of thermo-mechanical fatigue life prediction methods. The problem of their description is important in the development of a new design methodology for highly reliable pressure vessels.
The paper discusses the issue of the influence of the growth of the operation parameters of power plant components on the fatigue phenomena that take place under mechanical and thermal loading. The loading characteristics have been taken as measurements results in industrial conditions. Stress-strain characteristics of such components, which take into consideration the changeable properties of materials subject to fatigue, have been described. The hardening processes of the materials, which are assumed to be used in designed power plants, have also been discussed. The aim of the paper is the description of the mechanical behaviour of the power plant components taking into consideration the real industrial conditions of their operation and changeable mechanical properties of the components’ materials.
The paper presents the correlation between fatigue life determined under the conditions of low-cycle fatigue (LCF) and thermo-mechanical fatigue (TMF). Fatigue life values computed using own parameter P and results obtained based on the author’s own research and literature from the publications have been shown. The tests LCF and TMF have been performed for steels used for devices operated in the power engineering industry under the conditions of variable mechanical and thermal interactions X20CrMoV12.1 and P91.
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